The past several decades have seen a steady increase in the number of unmanned underwater robotic systems deployed for use in the ocean. These systems are also referred to as autonomous underwater vehicles (AUVs). All of these systems are equipped with energy systems including batteries to accomplish their respective mission. The conditions experienced by the batteries from its environment may affect the rate of charge. For example, for an autonomous vehicle that is underwater, the battery module may experience a lower temperature than room temperature. While batteries are typically capable of being discharged over a wide range of temperatures, batteries are more susceptible to damage caused by variations in temperature during the charging process. Early battery technology, such as lead acid and NiCd battery technology, typically have higher charging tolerances than newer battery technology. Hence, these types of batteries are able to be charged at temperatures below freezing. Fast charging of existing batteries is typically limited to 5 degrees Celsius to 45 degrees Celsius (41 degrees F. to 113 degrees F.). Some existing battery charging systems include a thermal blanket that heats a battery to an acceptable temperature. Consumer-grade lithium ion batteries cannot be charged below 0 degrees Celsius.
While it is possible to charge Li-ion batteries at low temperatures, acceptable current is very low and, therefore, the charge time is very long, which is not compatible with the desired pace of unmanned underwater vehicle operations. Excessive charging of a Lithium ion battery can cause damage to the battery, resulting reduction in battery capacity while making the battery unsafe. For example, in a Lithium ion battery, lithium ions move from the cathode to anode during charging and intercalate into the anode. In other words, the lithium ions are crammed in between the molecular gaps of the anode material's lattice, e.g., graphite. Graphite includes multiple graphene layers which reduce the anode's ability to convert the force from intercalation into internal stresses, resulting in significant volumetric strain on the anode (e.g., about 10-20% volume increase of the anode). This expansion can eventually weaken and puncture the membrane separating the anode from the cathode, causing a short of the battery cell and catastrophic failure. At cold temperatures, lithium ions do not efficiently intercalate into the graphite anode. Instead, the lithium ions plate the anode with metallic lithium (i.e., electroplate the anode), resulting in a substantial capacity reduction. Lithium plating of the anode also forms dendrites, which are tiny sharp tendrils of lithium metal that grow in the anode. These dendrites put pressure on the separating membrane (between the anode and cathode) as the anode expands and forces the dendrites into the membrane. Eventually, the membrane will fail, resulting in a short of the cathode and anode, which could further cause a catastrophic failure of the battery cell due to venting and possible ignition of flammable electrolyte.
Accordingly, there is a need for procedures and mechanisms such that a battery system, especially for an underwater vehicle subjected to low temperatures, can be protected against unacceptable damage during charging or discharging operations.